The goal of this study is to determine whether there are adverse health consequences of chronic low-level pesticide exposure in children. We have selected a population who are likely to be most at risk, farmworker children and others living in agricultural communities. These children are exposed to pesticides through the usual pathways such as ingestion of foods with pesticide residues, but in addition, ar exposed to the take- home exposures of farmworker parents and pesticide drift from nearby fields. If there are health consequences of low-level pesticide exposure, they should be most evident in this more heavily exposed population. Specifically, we will determine the relationship between neurodevelopment, growth, and the occurrence of asthma and chronic low-level pesticide exposure during the prenatal and/or postnatal periods. Over a one-year period, we will enroll a cohort of 550 pregnant women who receive care from either of two community clinics who primarily provide care to farmworkers, and we will follow their children up until age 3. The child will undergo neurobehavioral evaluations focussing on behaviors likely to be affected by chronic pesticide exposure during the neonatal period and at 6, 12, 24, and 26 months. At each medical visit, the child will be measured and weighed to assess growth patterns. At age 3, we will determine whether or not the child has active asthma. In an exploratory nested case-control study, we will also assess the effects of exposures to common environmental antigens and endotoxins on the development of Th-2 lymphocyte phenotypes and specific IgE and on asthma prevalence at age 3 years. Maternal urine samples collected during pregnancy and child urine samples will be measured for organophosphate metabolites. Home dust samples will be obtained during the prenatal period and at 6 and 12 months to measure common asthma-associated environmental antigens and endotoxin. Mothers will be interviewed pre- and postnatally about their habits, demographic characteristics, migration patterns, and work and medical history. At each well-baby visit, the mother will be questioned briefly about her child's development and illnesses (respiratory disease), breast- feeding, child care, and work. Low-level chronic pesticide exposure to children may be related to poor neurodevelopment and growth based on substantial evidence in developing rodents; and, it is biologically plausible that it is related to the occurrence of asthma through the dysregulation of the autonomic control of airways. To date, there is no research in developing humans.